need to support "keystone discover"

Steps to reproduce:

keystone --debug --username admin --password secrete --tenant_name admin --auth-url http://127.0.0.1:5000/v2.0/ discover

Expected:

Revelation

Actual:

$ keystone --debug --username admin --password secrete --tenant_name admin --auth-url http://127.0.0.1:5000/v2.0/ discover
connect: (127.0.0.1, 5000)
send: 'GET /v2.0/ HTTP/1.1\r\nHost: 127.0.0.1:5000\r\naccept-encoding: gzip, deflate\r\naccept: application/json\r\nuser-agent: python-keystoneclient\r\n\r\n'
reply: 'HTTP/1.1 200 OK\r\n'
header: Content-Type: application/json; charset=UTF-8
header: Content-Length: 2
header: Date: Tue, 14 Feb 2012 18:59:11 GMT
Unrecognized response from http://127.0.0.1:5000/v2.0/